Let's start with a very simple example. Suppose someone borrowed $100 from you today and offered to pay the loan back in full one year from today. Is $100 one year from now equal to $100 now? Of course not, because you could put that $100 in a simple savings account or some other interest bearing investment and have somewhat more than $100 next year. So, while you might let a friend have an interest free loan, it's probably not the way you would do business in general, at least not for long.